Conventional ultrasound imaging systems comprise an array of ultrasound transducer elements which transmit an ultrasound beam and receive the reflected beam from the object being studied. For medical ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved to scan the object.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound energy reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delays (and/or phase shifts) and gains to the signal from each receiving transducer element.
Such scanning comprises a series of measurements in which the steered ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received and stored. Typically, transmission and reception are steered in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
Ultrasonic imaging systems are known in which each transducer element is served by an individual analog channel followed by an analog-to-digital converter and one delay chip. Thus, a 128-channel system requires 128 delay chips and all of their associated memory and bus components. The delay chips introduce the time delays required for time delay beamforming. In the receive mode, the signals from all of the transducer elements are time delayed and then summed to form the summed signal representing all of the reflections from a point located at the desired range and steering angle.
It is widely accepted that a two-dimensional array would be advantageous in medical ultrasound imaging. Such an array would be steerable in both the azimuth and elevation directions. One of the limitations on the practicality of two-dimensional arrays is the electronic channel count. Simple brute force extension of conventional systems to such large systems is not practical. Increasing the number of connections to the transducer elements through the coaxial cable to the probe becomes prohibitive. Increasing the electronics of conventional beamforming systems by a factor of four or eight would be expensive and excessively power consuming. By duplexing, it is possible to double the number of effective channels; however, there is a need for further reduction in the number of channels needed to achieve a practical two-dimensional array.
The appearance of landmark papers by T. T. Taylor on the synthesis of linear and circular apertures (in 1955 and 1960) has revolutionized the design procedures for many radars of linear, rectangular or circular apertures which need sidelobe controls. Radar engineers find it fairly routine to incorporate Taylor synthesis in their designs. The Taylor method provides a nearly ideal pattern for realizable illumination of the aperture, and removes the deficiencies of classical Chebyshev arrays. Further, the method has found application in related fields, e.g., pulse compression or filter design, where windowing is essential. The success of the method is due primarily to simplicity of the procedure. The design engineer does not need to investigate the exact mathematical foundation on which the theory is based. It is such precise mathematical analysis that has led to the success of this theory, which uses special functions and asymptotic analysis.
Elliptical apertures and the corresponding analysis have not received great attention despite the wide potential application of elliptical aperture synthesis in modern radar applications, modern communication applications and reflector antennas. This may be due to the unavailability of simple design procedures for synthesis of elliptical arrays or apertures.
Taylor synthesis is outlined in radar handbooks. Advanced books on antenna and radar describe Taylor synthesis for linear arrays as well as for circular arrays to various degrees of sophistication. Some literature for the design of reflectors for communication with projected elliptical shape for earth coverage has appeared. Taylor synthesis can be extended to reduce the number of electrical channels while increasing performance, thereby enhancing imaging capability without additional hardware.